1. Field of the Invention
The present invention relates to a radio communication apparatus and method which perform frequency channel assignment in radio communication.
2. Description of the Related Art
In a cellular radio communication system in which a plurality of base stations are arranged to perform radio communication between themselves and mobile stations, overlaying the communication range of a given base station with that of another adjacent base station to some extent makes it possible to provide two-dimensional communication services for the mobile stations.
There is known a scheme of assigning different frequencies to a given base station and its adjacent base station to prevent communication between the given base station and the first mobile station and communication between the adjacent base station and the second mobile station from interfering with each other when the mobile stations are located in an overlapping area between the communication ranges of the two base stations (see JP-A 2002-271844 (KOKAI)). Although there is available a scheme in which when base stations are installed, available frequency channels are assigned to the respective base stations. In this case, however, it is necessary to install and design base stations in consideration of how the communication ranges to which frequencies are assigned overlap each other.
There is known a technique called DCA (Dynamic Channel Assignment) in which when starting communicating with a mobile station, each base station searches for an unused frequency channel which satisfies conditions, e.g., that the interference level is equal to or less than a predetermined value, and assigns the found frequency channel to the mobile station (see, for example, Shuichi Sasaoka, “Mobile Communication”, Ohmsha, p. 153 (section 6.6.2). Using DCA eliminates the necessity to assign frequency channels to the respective base stations in advance. This eliminates necessity to strictly install and design base stations.
The most basic scheme is a method of repeating the operation of selecting an arbitrary channel on a system band, assigning it to a given base station if it is not used, and arbitrarily selecting another channel if the arbitrary channel is used. It is also conceivable to use a method of making each base station measure an interference state for each channel, calculate a priority level corresponding to the interference amount, and search channels with high priority levels for an available channel.
When a received signal is to be demodulated by digital signal processing, the desired signal received at a radio frequency is converted into a baseband signal first, and the signal is then converted into a digital signal by an A/D converter (analog-to-digital converter). The smaller the bit width of the A/D converter, the better in terms of circuit size and power consumption. If, however, the A/D converter receives a signal from a base station B which is much larger than a desired signal from a base station A, since the desired signal component is very small, the A/D converter requires an extra number of bits. If, for example, the reception power difference is 24 dB, the A/D converter requires an extra bit width corresponding to four bits.
In order to avoid this, a receiver incorporates a filter in accordance with the bandwidth of a desired signal to remove channels other than the desired signal by using the filter, thereby inputting a signal component which is mostly the desired signal to the A/D converter.
Assume that DCA is applied to a system in which the bandwidth of a desired signal changes in accordance with a transmission rate. In this case, a problem arises when filters are mounted in the system. In the simplest case, a plurality of filters corresponding to the bandwidth of a desired signal may be mounted in the system, or a variable filter which can be adjusted in accordance with the bandwidth of a desired signal may be mounted in the system. This, however, increases the mount area.
It is also conceivable to use a method of mounting only one filter having the widest bandwidth possible and extracting only a desired signal by performing digital signal processing after A/D conversion if the bandwidth of a desired signal is smaller than that of the filter. In channel assignment by DCA, since a channel used by another base station may exist in the filter band, it is assumed, in particular, that a mobile station is located near base stations belonging to different carriers, as described above. In order to cope with this situation, the A/D converter needs to have an extra number of bits. This increases the mount area or power consumption of the A/D converter.